Some hidden gems in the DMG I've been dying to share for years. Here goes:
Page 9: Grab a set of polyhedral dice and arrange them exactly as they appear in the illustration in the right-hand column. Add the numbers showing face up (1+4+4+6=15). Notice that the top number on the d20 is blank. Now roll the d20. If you get a 15 the first time, give 5,000 XP to the player character of your choice. "15" is also the number of the next page containing a hidden Easter egg.
Page 15: Three dictators are mentioned as having Charisma 18. If you can name them without looking at the entry, you personally gain Charisma 18 but will be stabbed, shot, and poisoned before your next birthday.
Page 21: In the Trampier illustration of kobolds fighting a dragon, the kobolds' bodies spell out "E GARY GYGAX". Start with the kobold falling off the dragon's back (his arm and legs make an "E") and read counter-clockwise.
Page 35: The cartoon captioned "Dave, get the barbarian in the corner another drink, quick!" is a thinly-disguised reference to contemporary politics. From left to right the figures are Nixon speechwriter David Gergen, recently ousted President Gerald Ford, and Snake Plissken. The hatchet in the wall represents the angry mood of the electorate after the disgrace of the Nixon administration, and the "barbarian in the corner" is Jimmy Carter. If you look at the beer stein upside down, the lines on it read "JUU", which is code for the activist chant "Justice to You" (J2U). Snake Plissken represents the everyman, wronged by his government, now dispensing justice to politicians in disfavor.
Page 42: When generating character ability scores using a straight 3d6 roll, arrange the dice on the three magic circles and press lightly. The dice will warm slightly and will produce above average results for the next six rolls.
Page 99: A recipe for apple brown betty is inserted between two paragraphs in the Sample of Play script to see if anyone ever read it. The recipe calls for margarine in place of butter, a common substitution in midcentury American kitchens.
Page 170-173: Write down the name of each of the five monster species fought by the characters in Dave Sutherland's illustrations on these pages. The letters can be rearranged to spell "A Database Drivelling Old Monk Troll".
Page 187: In the Trampier Satyr illustration, the hairs on the right forearm (left as you look at it) clearly spell out "M. Poussin", an homage to Monsieur Nicolas Poussin, a French painter known for his emphasis on strong line and mythological subject matter.